The present invention relates to an imaging method and an imaging apparatus utilizing nuclear magnetic resonance (hereinafter referred to as "NMR"), and particularly to a method of forming an image having visually the highest signal-to-noise ratio within a limited period of measuring time.
An X-ray CT and an ultrasonic wave imaging apparatus have heretofore been widely used to noninvasibly inspect the internal structure such as head and belly of a human body. In recent years, it has been succeeded in performing the same inspection by utilizing the nuclear magnetic resonance phenomenon, making it possible to obtain data that were not obtainable with the X-ray CT and the ultrasonic wave imaging apparatus. In the imaging apparatus which utilizes the nuclear magnetic resonance phenomenon, signals from an object must be separated and discriminated so as to be corresponded to each of the portions of the object. One of the methods consists of applying a gradient field to the object so that each of the portions of the object is placed in a dissimilar static field. Namely, the portions of the object exhibit different resonance frequencies or different phase-encoding amounts, making it possible to obtain the data of positions.
The fundamental principle thereof has been disclosed in the Journal of Magnetic Resonance, Vol. 18, 1975, pp. 69-83 and in the Journal of Physics in Medicine and Biology, Vol. 25, 1980, pp. 751-756, and is not described here.
In such an imaging method, the signals are often added up together to improve the quality of the image.
That is, the whole signals necessary for reconstructing an image are repetitively measured in order to improve the signal-to-noise ratio of the signals. If the noise detected in each measurement does not have a correlation, the signal-to-noise ratio is improved in proportion to one-half power of the number of times of the measurement.
The measuring time, however, increases with the increase in the number of times of addition, and whereby a person being inspected suffers increased pain and the quality of image deteriorates to a degree that is not negligible as the person being inspected moves. Summary of the Invention
An object of the present invention is to improve the quality of image without permitting the measuring time to be so lengthened.
The above object is accomplished by effecting the addition of signals not for the whole signals that are necessary for reconstructing the image but by effecting the addition of signals for only part of the signals.
If mentioned more concretely, the invention deals with an imaging method called spin warp method in which NMR signals are measured while applying a phase-encoding gradient field in a predetermined direction and then applying a read-out gradient field in another direction different from the above predetermined direction, the signals are repetitively measured while changing the phase-encoding amount, and the thus obtained data train is subjected to the two-dimensional inverse Fourier transformation with the phase-encoding amount and the lapse of time during the measurement as variables, in order to reconstruct the magnetization distribution image of the object, wherein the measurement under a particular phase-encoding amount is repeated a plural number of times, a mean value is calculated from the obtained plurality of NMR signals, and the mean value is used for reconstructing the image.
As described above, the measurement is repeated a plural number of times not for all of the measured signals but for a particular phase-encoding amount only. The measurement is then averaged to obtain an image having reduced noise over a particular space frequency region without permitting the whole measuring time to be unnecessarily lengthened.